Step 1: An acid is a proton donor; to prove phenol is acidic, show reactions where the phenolic \(O-H\) hydrogen leaves.
Step 2 (Test with base): With aqueous NaOH, phenol is neutralised to the salt sodium phenoxide: \(C_6H_5OH + NaOH \rightarrow C_6H_5O^{-}Na^{+} + H_2O\). Only acids form salts with alkalis this way.
Step 3 (Test with active metal): With sodium metal it evolves hydrogen gas: \(2C_6H_5OH + 2Na \rightarrow 2C_6H_5ONa + H_2\). Liberation of \(H_2\) by a reactive metal is a standard sign of an acidic \(O-H\).
Step 4: Since both reactions remove the hydroxyl proton, they confirm the acidic nature of phenol.